WO2022210749A1

WO2022210749A1 - Spot welded joint and method for manufacturing spot welded joint

Info

Publication number: WO2022210749A1
Application number: PCT/JP2022/015612
Authority: WO
Inventors: 千智吉永; 正則泰山; 蒼紫川合; 松男茅野
Original assignee: 日本製鉄株式会社
Priority date: 2021-03-30
Filing date: 2022-03-29
Publication date: 2022-10-06
Also published as: US20240307992A1; JPWO2022210749A1; JP7453600B2

Abstract

Provided is a spot welded joint, in which, in a cross section in a plate thickness direction of a plate set in which two or more steel plates are stacked, the steel plates including at least one steel sheet having a C content of 0.280-0.700 mass%: the average of the short diameters of prior austenite particles, the long diameters thereof, and ratios (long diameter/short diameter) of the long diameters to the short diameters is in the range 1.0-1.5 in a melting boundary region, which is a region extending 1 mm inward from a melting boundary at a nugget end section; and the number density of an iron carbide having, at the melting boundary region, an average circle equivalent diameter of 30 nm or greater is 3.0×106×C content (mass%) or greater per 1 mm2. The plate set in which the C content of at least one steel plate is 0.280-0.700 in terms of mass% is subjected to spot welding under specific conditions, and subsequently, tempering thereof is performed under specific conditions.

Description

Spot-welded joint and method for manufacturing spot-welded joint

The present disclosure relates to spot welded joints and methods of manufacturing spot welded joints.

Spot welding is mainly used in processes such as assembly of car bodies and attachment of parts.
In recent years, in the automotive field, there has been a growing demand for lighter vehicle bodies to achieve better fuel efficiency and reduced _CO2 emissions, and higher rigidity for improved collision safety. For this reason, there is a growing need to use high-strength steel sheets for vehicle bodies and parts.
On the other hand, in high-strength steel sheets, the carbon equivalent (Ceq) of the base material is large in order to achieve the strength, and in spot welding, the welded part is rapidly cooled immediately after heating, so the welded part becomes a martensitic structure. Hardness increases and toughness decreases in welds and heat affected zones.

As a method for improving the toughness of spot welds and ensuring joint strength, there has been proposed a method in which post-heating energization is performed after main energization.
For example, in Patent Document 1, as a spot welding method by three-step energization, a plate assembly in which two or more steel plates are superimposed is sandwiched between a pair of electrodes, and a resistance spot welding method is performed by energizing while applying pressure. Then, a main energizing step is performed in which current is applied at a current value I _w (kA), and then, as a post-tempering heat treatment step, cooling is performed for the cooling time t _ct (ms) shown in Equation (1), and then Equation (2 ) at a current value I _t (kA) shown in the formula (3) for the energization time t _t (ms) shown in the equation (3),
800≦t _ct Formula (1)
0.5×I _w ≦I _t ≦I _w Formula (2)
500≦t _t Expression (3)
At least one steel plate in the set of plates,
0.08≦C≦0.3 (% by mass),
0.1 ≤ Si ≤ 0.8 (% by mass),
2.5≦Mn≦10.0 (% by mass),
P ≤ 0.1 (% by mass)
A resistance spot welding method is disclosed having a composition containing the balance Fe and unavoidable impurities.

In addition, Patent Document 2 discloses a method of lap-welding high-strength steel plates containing 0.15% by mass or more of carbon and having a tensile strength of 980 MPa or more, wherein the spot welding step is performed as the first step to form a nugget. The first energization step, the first energization step followed by a cooling step in which no energization is performed, and the cooling step followed by the second energization step in which the nugget is softened are performed. _1. When the current in the second energization step is I ₂ , I ₂ /I ₁ is set to 0.5 to 0.8, and the time tc (sec) of the cooling step is set according to the steel plate thickness H (mm). , the range of 0.8 × tmin or more and 2.5 × tmin or less calculated by the following formula (1), and the energization time t2 (sec) of the second energization step is set to 0.7 × tmin or more Disclosed is a spot welding method in which a spot welding joint is obtained by welding with a range of 2.5 × tmin or less, and welding with a larger electrode pressure after the cooling process than the electrode pressure until the first energization process. It is
tmin=0.2×H2 ( ¹ )

Further, in Patent Document 3, two or more thin steel plates spot-welded to each other,
a nugget formed on the joining surface of the thin steel plate;
has
At least one of the two or more thin steel sheets is a high-strength steel sheet having a tensile strength of 750 MPa to 1850 MPa, and a carbon equivalent Ceq represented by the following formula (1) is 0.22 mass% to 0.55. % by mass,
In the nugget outer layer area excluding the similar shape area of 90% of the outer shape of the nugget in the nugget,
The microstructure consists of a dendrite structure with an average arm spacing of 12 μm or less,
A spot-welded joint of high-strength steel sheets is disclosed, characterized in that the average grain size of carbides contained in the microstructure is 5 nm to 100 nm and the number density is 2×10 ⁶ pieces/mm ² or more.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] (1)
([C], [Si], [Mn], [P] and [S] indicate the contents (% by mass) of C, Si, Mn, P and S, respectively.)

Patent Document 1: International Publication No. 2019/156073 Patent Document 2: International Publication No. 2014/171495 Patent Document 3: International Publication No. 2011/025015

By increasing the carbon content of the steel plate used for spot welding, it is possible to increase the strength of the joint base material (steel plate). However, the strength of spot-welded joints decreases with high Ceq materials.
For example, in Patent Document 1, it is essential to use a steel plate with a C content of 0.08 to 0.3%, and as a comparative example, a steel plate with a C content of more than 0.3% is used. It is described that the joint strength decreases when the However, in the examples of Patent Document 1, steel sheets with a C content of 0.2% or less are used, and those with a C content of 0.28% are comparative examples.
In order to improve the collision performance of spot-welded members, it is desirable to produce welded joints with high joint strength and welded joints with high joint strength.

An object of the present disclosure is to provide a spot-welded joint whose joint strength is greatly improved compared to the case where resistance spot welding is performed by single current flow to a plate set including steel plates having a relatively high carbon content.

The present disclosure provides a method for manufacturing a spot-welded joint that can greatly improve the joint strength compared to the case of performing resistance spot welding by single current, even when using a plate set containing a steel plate with a relatively high carbon content. The purpose is to

The gist of the present disclosure for achieving the above object is as follows.
<1> A spot-welded joint of a plate set in which two or more steel plates including at least one steel plate having a C content of 0.280% by mass or more and 0.700% by mass or less are superimposed,
In the cross section in the plate thickness direction of the plate assembly passing through the center of the nugget, the length of the major diameter to the minor diameter of the prior austenite grains in the molten boundary region up to 1 mm inside from the molten boundary of the nugget edge corresponding to the plate interface. When the average ratio (major axis/minor axis) is in the range of 1.0 to 1.5, and the C content (% by mass) of the steel sheets constituting the plate set is C, the circle in the fusion boundary region A spot-welded joint in which the number density of iron-based carbides having an equivalent diameter of 30 nm or more is 3.0×10 ⁶ ×C or more per 1 mm ² .
However, if the C content for calculating the lower limit of the number density of the iron-based carbides is not the same for all the steel plates that make up the set, the C of each steel plate that makes up the set A weighted average of values obtained by multiplying the content by the plate thickness ratio of each steel plate to the total thickness of the plate assembly.
<2> The spot-welded joint according to <1>, wherein the C content of all the steel plates that form the set of plates is more than 0.300% by mass.
<3> When the C content (% by mass) in the steel plate with the highest C content in the plate set is [C], the tensile strength (MPa) of the steel plate with the highest C content is 1800 × [C] The spot welded joint according to <1> or <2>, which is +250 or more.
<4> The number density of iron-based carbides having an equivalent circle diameter of 30 nm or more in a region within 500 μm from the nugget end of the heat-affected zone existing around the nugget end is 1.0× per 1 mm ² The spot welded joint according to any one of <1> to <3>, which is 10 ⁶ ×C or more.
<5> The spot welded joint according to any one of <1> to <4>, wherein the residual stress at the center of the nugget is less than 90 MPa.
<6> A plate set in which two or more steel plates including at least one steel plate having a C content of 0.280% or more and 0.700% by mass or less are superimposed with a pair of electrodes in the plate thickness direction A first energizing step of energizing at a current value I ₁ (kA) while sandwiching and applying pressure;
After the first energization step, a first non-energization step in which the time _tc1 of 20 ms or more and 200 ms or less is not energized;
After the first non-energization step, a second energization step of energizing at a current value I ₂ (kA) that satisfies the following formula (1) and a time t ₂ (ms) that satisfies the following formula (2);
0.60≦I ₂ /I ₁ ≦1.10 (1)
50≦t ₂ ≦1000 (2)
After the second energization step, after the time t _c2 (ms) that satisfies the following formula (3) has passed, the tempering temperature is 350 ° C. or higher at the energized position, and the tempering temperature is calculated by the following formula (A). A tempering step of performing tempering under conditions where the tempering parameter H is 8000 or more and 18000 or less;
t _c2 >3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100 (3)
H = T x (log _HT + (17.7 - 5.8 x [C])) (A)
A method of manufacturing a spot welded joint, comprising:
The Ms in the formula (3) means the Ms point calculated by substituting the mass % of each element contained in the steel sheets forming the set of plates into the element symbol in the following formula (4). However, if all the steel plates that make up the set of plates do not have the same composition, the total thickness of the set of plates is added to the Ms point calculated for each steel plate by the above formula (4) for all the steel plates that make up the set of plates. Substitute the weighted average Ms point of the value obtained by multiplying the plate thickness ratio of each steel plate to the formula (3).
Ms (°C) = 561-474 x C-33 x Mn-17 x Ni-17 x Cr-21 x Mo (4)
T in the formula (A) means the tempering temperature (K) in the vicinity of the edge of the nugget formed by the energization, _tHT means the tempering time (s), and [C] means C content in the plate set. It means the C content (mass%) in the steel sheet with the highest amount.
<7> Manufacture of a spot welded joint according to <6>, wherein in the tempering step, the tempering is performed using a heating means selected from the group consisting of a furnace, a laser, a tempering iron, a hot plate, and high-frequency induction heating. Method.
<8> When the value calculated by the following formula (B) is A _c1 (° C.), in the tempering step, the tempering temperature T is set to (A _c1 −30)° C. or less. The method for manufacturing a spot-welded joint according to <6> or <7>, in which the return is performed.
A _c1 (° C.)=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B (B)
The mass % of each element contained in the steel sheets forming the set is substituted for the symbol of the element in the formula (B). However, if all the steel plates that make up the set of plates do not have the same composition, A _c1 calculated by the above formula (B) for each steel plate for all the steel plates that make up the set of plates is added to the total thickness of the set of plates. The above (A _c1 -30) is determined based on the weighted average A _c1 of the values obtained by multiplying the thickness ratio of each steel plate to .
<9> The method for manufacturing a spot-welded joint according to any one of <6> to <8>, wherein the carbon content of all the steel plates that make up the set of plates is more than 0.300% by mass.
<10> When the C content (mass%) in the steel plate with the highest C content in the plate set is [C], the tensile strength (MPa) of the steel plate with the highest C content is 1800 × [C] The method for producing a spot-welded joint according to any one of <6> to <9>, which produces a spot-welded joint of +250 or more.
<11> The method for manufacturing a spot-welded joint according to any one of <6> to <10>, wherein t _c2 is 9000 msec or less.
<12> Any one of <6> to <11>, wherein in the first energizing step, the first non-energizing step, and the second energizing step, the pressure applied to the plate assembly by the pair of electrodes is constant. A method for manufacturing a spot-welded joint according to .

According to the present disclosure, there is provided a spot-welded joint whose joint strength is greatly improved compared to the case where resistance spot welding is performed by single current flow to a plate set including steel plates having a relatively high carbon content.

According to the present disclosure, there is provided a method for manufacturing a spot-welded joint that can greatly improve the joint strength compared to the case of performing resistance spot welding by single current, even when using a plate assembly containing a steel plate having a relatively high carbon content. provided.

FIG. 4 is a diagram showing the relationship between spot welding performed on superimposed steel plates and the CTS (cross tensile strength) of the joint. It is an SEM-EBSD analysis image near the nugget after spot welding, (A) is a case where only a single current is applied, and (B) is a case where a second current is applied after the single current. It is a figure which shows the cross section of the plate|board thickness direction around a nugget. It is an example of the structure of the nugget edge when a spot-welded joint is manufactured by single current flow, and is a diagram showing (A) an SEM-EBSD analysis image and (B) prior austenite grain boundaries. FIG. 4 shows an example of the structure of the nugget edge of the spot-welded joint according to the present disclosure, showing (A) SEM-EBSD analysis image and (B) prior austenite grain boundaries. FIG. 4 is an example of the structure of the nugget end portion of the spot-welded joint, showing iron-based carbides (white portions). FIG. 3 schematically illustrates a combination of spot welding and tempering in a method of manufacturing a spot welded joint according to the present disclosure; FIG. 2 is a diagram schematically showing an example of a nugget and a heat-affected zone (HAZ) formed when resistance spot welding is performed on a plate set in which two steel plates are superimposed; FIG. 4 is a diagram showing the relationship between the Ms point and the time required for cooling to the Ms point after segregation relaxation. FIG. 5 is a diagram showing an example of a temperature history obtained by thermal conduction analysis of the vicinity of the nugget edge when tempering is performed using a spot welder. FIG. 11 is a diagram showing average temperature changes when the temperature history shown in FIG. 10 is divided into ranges not exceeding 50° C.;

An embodiment that is an example of the present disclosure will be described below.
In addition, in this disclosure, "%" display of the content of each element means "% by mass". In addition, in the present disclosure, unless otherwise specified, a numerical range represented using "to" means a range including the numerical values described before and after "to" as lower and upper limits. In addition, a numerical range when "more than" or "less than" is attached to a numerical value described before and after "to" means a range that does not include these numerical values as lower or upper limits.
In the numerical ranges described step by step in the present disclosure, the upper limit of one step of the numerical range may be replaced with the upper limit of another step of the numerical range or the values shown in the examples. good. In addition, in the numerical ranges described step by step in the present disclosure, the lower limit of one step of the numerical range is replaced with the lower limit of another step of the numerical range or the value shown in the example. may
In addition, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

The present inventors have found a method that can improve the joint strength (cross tensile strength: CTS) when resistance spot welding is performed even if the C content of the steel plate is more than 0.150% by mass, particularly 0.280% or more. studied intensively. Fig. 1 shows two types of steel sheets with a C content of 0.34% and a P content of 0.015% (normal P material) and 0.0007% (extremely low P material). It shows the relationship between the CTS of the joint where two sheets are superimposed and resistance spot welded. Components other than the amount of C and the amount of P are common (S: 0.0008%, Si: 0.25%, Mn: 1.25%). In addition, "single energization" is resistance spot welding performed by one energization to form a nugget on the sheet assembly, and "tempering energization" is to perform a single energization to form a nugget on the sheet assembly, and then the nugget is welded. It means that post-energization (tempering energization) corresponding to annealing treatment for softening was performed. "Three-step energization" means that after single energization for forming a nugget, energization with a current value larger than that for tempering energization was performed, and then tempering energization was performed. "Two-step energization + furnace tempering" means that after single energization for forming a nugget, energization with a current value higher than that for tempering energization was performed, and then tempering was performed using a tempering furnace.

When investigating the joint strength using steel sheets with different compositions, it was found that the joint strength achieved when the tempering current was applied was higher with the ultra-low P than with the normal P content. . Therefore, when ordinary P is subjected to tempering energization or tempering in a furnace, if energization is added for the purpose of segregation relaxation, even higher strength can be obtained, but this segregation relaxation effect alone cannot explain. It was found that the joint strength increased as the
The reason for this is thought to be that the re-energization after forming the nugget has not only the effect of alleviating the segregation but also the effect of changing the shape of the prior austenite grains. FIG. 2 is an image obtained by SEM-EBSD analysis of a spot-welded nugget and its vicinity. (A) is a case where only a single energization is performed, and (B) is a case where the second stage energization is performed for 0.1 second after the single energization (no tempering energization). Looking at the large-angle grain boundaries of 15 degrees or more, in (B), regular grains, which are rarely seen in (A), are observed near the nugget edge (near the melt boundary) within the nugget. It is considered that the change in the grain size shape is caused by the fact that the steel is once solidified, then heated again to undergo delta transformation, and then cooled again to undergo γ transformation. In a high-C steel with a C content of more than 0.150%, particularly 0.280% or more, grain regulating is considered to be more important than segregation mitigation.

After such two _{energizations} , the welded portion was tempered using various heat sources. ]: When the C content (% by mass) of the steel plate is used, tempering is performed so that the tempering parameter H, which can be calculated from the temperature history of the nugget edge, is within a specific range, so that the joint by single current flow CTS was remarkably improved in comparison.
In general, the higher the C content, the higher the tensile strength of the steel sheet, but the toughness of the weld zone decreases and the joint strength decreases. In the case of steel sheets, it is important not only to relax segregation but also to regulate grain size. Then, even for a sheet set containing steel sheets with a C content of 0.280% or more and 0.700% or less, the energization step of forming nuggets under specific conditions and the grain size regulating energization step are combined. If spot welding is performed and after a specific time has elapsed, tempering is performed so that the value of the tempering parameter H is within a specific range, in the CTS test, the part where the stress in the peeling direction is most applied (inside the nugget It was found that the toughness in the vicinity of the nugget boundary) is improved, and the joint strength can be greatly improved.

Further, spot-welded joints using steel sheets with a relatively high carbon content were closely examined by cross-sectional observation of nuggets as shown in FIG. 3, CTS tests, and the like. FIG. 4 shows the structure of the nugget edge when a spot-welded joint is manufactured by single current flow, and FIG. (A) is an image obtained by SEM-EBSD analysis, and (B) shows grain boundaries of prior austenite grains. Compared with the prior austenite grains shown in FIG. 4(B), the prior austenite grains shown in FIG. 5(B) have regular grains with a smaller aspect ratio. In addition, FIG. 6 shows iron-based carbides (white portion) at the nugget edge.
Based on these analysis results, in spot welded joints using steel sheets with a C content of 0.280% or more and 0.700% by mass or less, from the fusion boundary at the nugget edge corresponding to the plate interface, It was found that in the fusion boundary region up to 1 mm inside, when the following (I) and (II) are satisfied, the CTS is significantly improved compared to the welded joint spot-welded by a single current.
(I) The average ratio of the major axis to the minor axis of the prior austenite grains (major axis/minor axis) is 1.0 to 1.5.
(II) The number density of iron-based carbides having an equivalent circle diameter of 30 nm or more is 3.0×10 ⁶ ×C or more per 1 mm ² where C is the C content (% by mass) of the steel sheet.

[Spot welding joint]
The spot welded joint according to the present disclosure will now be described in detail. In the present disclosure, "spot welded joint" is referred to as "welded joint" or simply "joint",
"Nugget edge corresponding to the plate interface" is "nugget edge",
"The area from the melting boundary to the inside 1 mm" is the "melting boundary area",
"Average ratio of major axis to minor axis of prior austenite grains (major axis / minor axis)" is "average aspect ratio of prior austenite grains",
"Iron-based carbide having an equivalent circle diameter of 30 nm or more" is "coarse iron-based carbide",
are sometimes called respectively.
For example, "In the cross section in the plate thickness direction of the plate assembly, the ratio of the major axis to the minor axis of the prior austenite grain in the region from the melt boundary of the nugget end corresponding to the plate interface to the inside 1 mm (long axis/short axis ) is referred to as the “average aspect ratio of the prior austenite grains at the nugget edge”, and “in the cross section in the plate thickness direction of the plate assembly, 1 mm inside from the melt boundary at the nugget end corresponding to the plate interface The number density of iron-based carbides having an equivalent circle diameter of 30 nm or more in the region up to the edge of the nugget may be referred to as "the number density of coarse iron-based carbides at the nugget edge".

(Average aspect ratio of prior austenite grains at nugget edge)
The welded joint according to the present disclosure has an average aspect ratio of prior austenite grains at the nugget edge in the range of 1.0 to 1.5. If the conditions of the method for manufacturing a spot-welded joint according to the present disclosure, which will be described later, are not satisfied, the austenite grains tend to be elongated in the solidification direction, the superimposed plates are weak against the force in the direction of peeling, and the joint strength is low. to degrade. Therefore, the average aspect ratio of the prior austenite grains at the nugget edge is set to 1.0 to 1.5, preferably 1.3 or less, and more preferably 1.2 or less.

In the present disclosure, the average aspect ratio of prior austenite grains at the nugget edge is specified as follows.
For example, in the images showing the prior austenite grain boundaries as shown in FIGS. 4B and 5B, the shape of each prior austenite grain is approximated to an ellipse by the least-squares method. In the ellipse approximation method, the major axis of each austenite grain and the area are used to calculate the minor axis of an ellipse having the major axis. In this elliptical shape, the aspect ratio of the prior austenite grains is calculated by dividing the dimension of the major axis by the dimension of the minor axis.
Specifically, cut in the plate thickness direction so as to pass through the center of the nugget, and observe the molten boundary region at the end of the nugget in the cross section with SEM-EBSD at a magnification of 50 times and an observation area of 0.25 mm ² as the former austenite grain boundary. measure the aspect ratio of Measurements are taken in the fusion boundary region of the nugget edge, and the average value thereof is taken as the average aspect ratio. The number of prior austenite grains in the fusion boundary region at the edge of the nugget for calculating the average aspect ratio is 15 or more. The aspect ratio of the prior austenite grain boundary may be measured in the molten boundary region at either end of the nugget, but if the prior austenite grain size is large and 15 or more cannot be measured, , the total observation area measured at both ends of the nugget shall be 0.25 mm ² or more, and the shape of the prior austenite grains contained therein shall be used. At this time, even if the grain contained therein is outside the range of 0.25 mm ² , it shall be used for the calculation.
In the case of a joint where three or more steel sheets are stacked and spot-welded, the measurement is performed at the edge of the nugget at the interface of the steel sheet with the highest carbon content. Measurement is taken at the nugget edge of the plate interface on the side with the higher carbon content above and below.

(Number density of iron-based carbides at nugget edge)
In the welded joint according to the present disclosure, the number density of iron-based carbides (coarse iron-based carbides) having an equivalent circle diameter of 30 nm or more at the nugget edge is 3.0×10 ⁶ ×C or more per 1 mm ² . When the number density of coarse iron-based carbides at the nugget edge is 3.0×10 ⁶ ×C pieces/mm ² or more, the tempering has progressed sufficiently and a high joint strength can be obtained. The number density of coarse iron-based carbides at the nugget edge is preferably 3.3×10 ⁶ ×C pieces/mm ² or more, more preferably 4.0×10 ⁶ ×C pieces/mm ² or more.
^On the other hand, if the number density of coarse iron ^- based carbides at the end of the nugget is too large, the toughness may be reduced. 0×10 ⁸ ×C pieces/mm ² or less.
For C, the C content (mass%) of the steel plates that make up the set is substituted, but if the C content of the steel plates that make up the set is different, , the weighted average of the values obtained by multiplying the thickness ratio of each steel plate to the total thickness of the set.

In the present disclosure, the number density of coarse iron-based carbides at the nugget edge is specified as follows.
In a cross section cut in the plate thickness direction so as to pass through the center of the nugget, the molten boundary region including the corresponding position of the nugget edge is mirror-polished, etched with nital, and then observed with SEM (magnification: 20000 times). The composition of the precipitates, which are considered to be iron-based carbides, is specified by EDS (Energy dispersive X-ray spectrometry). The iron-based carbides mentioned here mainly include cementite (Fe ₃ C), which is a compound of iron and carbon, and ε-based carbides (Fe _2-3 C). In addition to these iron-based carbides, compounds obtained by substituting Fe atoms in cementite with Mn, Cr, etc., and alloy carbides (M ₂₃ C ₆ , M ₆ C, MC, etc., where M is Fe and other metal element). Among these iron-based carbides, the number density of those having an equivalent circle diameter exceeding 30 nm may be measured in a field of view of 50 μm square or more at the edge of the nugget where the aspect ratio of the prior austenite grain boundary is measured.

(steel plate)
In the welded joint and the method for manufacturing the welded joint according to the present disclosure, at least one steel plate constituting the plate assembly may have a C content of 0.280% or more and 0.700% or less in mass%. . The number of steel plates constituting the set of plates is not particularly limited as long as it is two or more, and may be selected according to the application of the welded joint to be manufactured. The steel plate in the welded joint and the method for manufacturing the welded joint according to the present disclosure will be described below.

C: 0.280% or more and 0.700% or less C is an element that enhances the hardenability of steel and contributes to strength improvement. When spot welding is performed by stacking only steel plates with a C content of less than 0.280%, the joint strength can be ensured without applying the welded joint according to the present disclosure. The C content of at least one steel sheet is 0.280% or more. Preferably, the C content of all steel sheets constituting the welded joint according to the present disclosure is 0.280% or more, more preferably more than 0.300%, even more preferably 0.310% or more, even more preferably 0 0.330% or more, more preferably 0.350% or more.
However, if the C content exceeds 0.700%, the toughness is too low, and even if the welded joint according to the present disclosure is applied, only a low CTS can still be obtained, so the C content is 0.700% or less. . The C content is preferably 0.550% or less, more preferably 0.480% or less.

The balance other than C may be Fe and impurities, or may include optional components in place of part of Fe. Impurities are exemplified by components contained in raw materials such as ores and scraps, or components mixed in during the manufacturing process, and refer to components that are not intentionally included in the steel sheet. Preferred contents of elements other than C and Fe are described below. In addition, the components described below are impurities or arbitrary components, and the lower limit may be 0% or may be more than 0%.

P: 0.010% or less P is an impurity and an element that causes embrittlement. If the P content exceeds 0.010%, it is difficult to obtain joint strength, so the upper limit is preferably made 0.010%. More preferably, it is 0.009% or less.
It should be noted that the lower the P content, the better, but the lower the P content, the higher the P removal cost. In addition, according to the welded joint according to the present disclosure, as shown in FIG. 1, even when a steel plate with a normal P content is used, a steel plate with an extremely low P content is used to form a nugget by energization. After that, the CTS can be improved to the same level or more as when the tempering energization is performed. Therefore, the P content of the steel sheet does not need to be greatly reduced, and the lower limit of the P content may be 0.0005%.

S: 0.050% or less S, like P, is an impurity and an element that causes embrittlement. Also, S is an element that forms coarse MnS in steel, lowering the workability of the steel and also lowering the joint strength. If the S content exceeds 0.050%, it is difficult to obtain the required joint strength and the workability of the steel deteriorates.
The lower the S content, the better, but from the same viewpoint as P, the lower limit of the S content in the steel sheet may be 0.0003%.

Si: more than 0.10% Si is an element that increases the strength of steel through solid-solution strengthening and structural strengthening. If the Si content is 0.10% or less, the joint strength decreases, so it is preferable to set the lower limit to more than 0.10%. More preferably, it exceeds 0.80%.
On the other hand, if the Si content is too high, the workability is lowered and the joint strength is lowered, so the upper limit may be 3.5% or 3.0%.

Mn: 15.00% or less Mn is an element that increases the strength of steel. If the Mn content exceeds 15.00%, the workability deteriorates and the joint strength also decreases, so the upper limit is preferably set to 15.00%. 0.5 to 7.5% is more preferable in order to ensure the strength, workability, and joint strength of the steel sheet in a well-balanced manner. More preferably, it is 1.0 to 3.5%.

Al: 3.00% or less Al is an element that acts as a deoxidizer, stabilizes ferrite, and suppresses precipitation of cementite. Al is contained for deoxidizing and controlling the steel structure, but Al is easily oxidized. is also reduced, it is preferable to make it 3.00% or less. A more preferable upper limit is 1.2% from the viewpoint of ensuring workability.

N: 0.0100% or less N is an element that increases the strength of the steel sheet, but it is an element that forms coarse nitrides in the steel and degrades the formability of the steel. If the N content exceeds 0.0100%, the formability of the steel deteriorates and the joint strength remarkably decreases.
From the viewpoint of improving the cleanliness of the steel sheet, N may be 0%. From the viewpoint of production cost for reducing N, the lower limit may be 0.0001%.

Ti: 0.70% or less Ti is an element that forms precipitates and refines the steel sheet structure, and may be contained. In order to obtain the effect of inclusion, the content is preferably 0.001% or more. More preferably, it is 0.01% or more. On the other hand, if it is contained excessively, the manufacturability deteriorates, cracks occur during processing, and joint strength also decreases, so the upper limit is preferably 0.70%, more preferably 0.50% or less. be.

Nb: 0.70% or less Nb is an element that forms fine carbonitrides and suppresses coarsening of crystal grains, and may be contained. In order to obtain the effect of inclusion, the content is preferably 0.001% or more. More preferably, it is 0.01% or more. Excessive content impairs toughness and makes manufacturing difficult and causes a decrease in joint strength, so the upper limit is preferably 0.70%, more preferably 0.50% or less, or 0.30% or less. is.

V: 0.30% or less V is an element that forms fine carbonitrides and suppresses coarsening of crystal grains, and may be contained. In order to obtain the effect of inclusion, the content is preferably 0.001% or more. More preferably, it is 0.03% or more. An excessive content impairs the toughness, making it difficult to manufacture and also causes a decrease in joint strength. Therefore, the upper limit is preferably 0.30%, more preferably 0.25% or less.

Cr: 5.00% or less Mo: 2.00% or less Cr and Mo are elements that contribute to improving the strength of steel and may be contained. In order to obtain the containing effect, it is preferable to contain 0.001% or more of each. More preferably, it is 0.05% or more. However, if the Cr content exceeds 5.00% or the Mo content exceeds 2.00%, problems may occur during pickling and hot working, and joint strength may be reduced. , the upper limit of the Cr content is preferably 5.00%, and the upper limit of the Mo content is preferably 2.00%.

Cu: 2.00% or less Ni: 10.00% or less Cu and Ni are elements that contribute to improving the strength of steel and may be contained. In order to obtain the containing effect, it is preferable to contain 0.001% or more of each. More preferably, it is 0.10% or more. However, if the Cu content exceeds 2.00% and the Ni content exceeds 10.00%, problems may occur during pickling and hot working, and joint strength may be reduced. Therefore, the upper limit of the Cu content is preferably 2.00%, and the upper limit of the Ni content is preferably 10.00%.

Ca: 0.0030% or less REM: 0.050% or less Mg: 0.05% or less Zr: 0.05% or less Ca, REM (rare earth metal), Mg, and Zr are oxides after deoxidation and , is an element that refines sulfides present in the hot-rolled steel sheet and contributes to the improvement of formability, and may be contained. However, when the Ca content exceeds 0.0030%, the REM content exceeds 0.050%, and each Mg or Zr content exceeds 0.05%, the workability of the steel is reduced. . Therefore, it is preferable to set the upper limit of Ca content to 0.0030%, the upper limit of REM content to 0.050%, and the upper limit of each content of Mg and Zr to 0.05%.
In order to obtain the inclusion effect, it is preferable that the Ca content is 0.0005% or more, the REM content is 0.001% or more, the Mg content is 0.001% or more, and the Zr content is 0.001% or more.

"REM" is a generic term for a total of 17 elements including Sc, Y, and lanthanoids, and the content of REM refers to the total content of one or more elements in REM. REM is generally contained in misch metal. Therefore, for example, REM may be contained in the form of misch metal so that the total content of REM is within the above range.

B: 0.0200% or less B is an element that segregates at grain boundaries to increase grain boundary strength, and may be contained. In order to obtain the effect of inclusion, the content is preferably 0.0001% or more, more preferably 0.0008% or more. On the other hand, an excessive content impairs the toughness, making it difficult to manufacture and lowering the joint strength, so the upper limit is preferably 0.0200%, more preferably 0.010% or less.

In the welded joint according to the present disclosure, at least one steel plate in the plate set in which two or more steel plates are superimposed has a C content in mass% of 0.280% or more and 0.700% or less. Further, a desired element is selected from the above elements, and a steel sheet having a composition within the above range is used.
Steel plate
C: 0.280% to 0.700%,
Si: more than 0.10%,
Mn: 15.00% or less,
P: less than 0.010%,
S: 0.0100% or less,
Al: 3.00% or less, and N: 0.0100% or less,
It may be a steel sheet with the balance being iron (Fe) and impurities.
The steel plate of the above composition replaces part of the iron (Fe),
Ti: 0.70% or less,
Nb: 0.70% or less,
V: 0.30% or less,
Cr: 5.00% or less,
Mo: 2.00% or less,
Cu: 2.00% or less,
Ni: 10.00% or less,
Ca: 0.0030% or less,
REM: 0.050% or less,
Mg: 0.05% or less,
It may contain one or more elements selected from the group of Zr: 0.05% or less and B: 0.0200% or less.
The C content of all the steel plates constituting the set may be 0.280% or more and 0.700% or less, and some of the steel plates in the set may have a C content of less than 0.280% or 0.280%. It may be greater than 700%.

The plate thickness of each steel plate that constitutes the plate assembly is not particularly limited, but for example, a plate thickness of 0.5 to 3.5 mm can be mentioned.
Also, the total thickness t of the board assembly is not particularly limited, but it is, for example, 1.5 to 8.0 mm.

Although the use of the welded joint according to the present disclosure is not particularly limited, it is considered particularly effective for body parts, for example.

[Manufacturing method of spot welded joint]
Although the method for manufacturing the spot-welded joint according to the present disclosure is not particularly limited, two or more steel plates including at least one steel plate having a C content of 0.280% or more and 0.700% or less are superimposed. After a predetermined time t _c2 (ms) has passed after the first energization, the first non-energization, and the second energization at a specific current value and time, a specific A method of tempering under certain conditions is mentioned. According to such a method, the CTS can be significantly improved as compared with the case where resistance spot welding is performed by single current flow, and the spot welded joint according to the present disclosure can be suitably manufactured. An example of a preferred method for manufacturing a spot-welded joint according to the present disclosure (sometimes referred to as a "method for manufacturing a spot-welded joint according to the present disclosure") will be described in detail below.

That is, the method for manufacturing a spot-welded joint according to the present disclosure (sometimes simply referred to as the “method for manufacturing a welded joint” in the present disclosure) has a C content of 0.280% or more and 0.280% or more by mass%. A plate assembly in which two or more steel plates including at least one steel plate of 700% by mass or less are superimposed is sandwiched between a pair of electrodes in the plate thickness direction and pressed while applying current at a current value I ₁ (kA). 1 energization step;
After the first energization step, a first non-energization step in which the time _tc1 of 20 ms or more and 200 ms or less is not energized;
After the first non-energization step, a second energization step of energizing at a current value I ₂ (kA) that satisfies the following formula (1) and a time t ₂ (ms) that satisfies the following formula (2);
0.60≦I ₂ /I ₁ ≦1.10 (1)
50≦t ₂ ≦1000 (2)
After the second energization step, after the time t _c2 (ms) that satisfies the following formula (3) has passed, the tempering temperature is 350 ° C. or higher at the energized position, and the tempering temperature is calculated by the following formula (A). A tempering step of performing tempering under conditions where the tempering parameter H is 8000 or more and 18000 or less;
t _c2 >3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100 (3)
H = T x (log _HT + (17.7 - 5.8 x [C])) (A)
including.
The Ms in the formula (3) means the Ms point calculated by substituting the mass % of each element contained in the steel sheets forming the set of plates into the element symbol in the following formula (4). However, if all the steel plates that make up the set of plates do not have the same composition, the total thickness of the set of plates is added to the Ms point calculated for each steel plate by the above formula (4) for all the steel plates that make up the set of plates. Substitute the weighted average Ms point of the value obtained by multiplying the plate thickness ratio of each steel plate to the formula (3).
Ms (°C) = 561-474 x C-33 x Mn-17 x Ni-17 x Cr-21 x Mo (4)
T in the formula (A) means the tempering temperature (K) in the vicinity of the edge of the nugget formed by the energization, _tHT means the tempering time (s), and [C] means C content in the plate set. It means the C content (mass%) in the steel sheet with the highest amount.

FIG. 7 is a diagram schematically showing spot welding (current and time) and tempering in the method of manufacturing a spot welded joint according to the present disclosure. A method for manufacturing a welded joint according to the present disclosure is a plate in which two or more steel plates including at least one steel plate having a C content of 0.280% or more and 0.700% or less in mass% 7, the tempering temperature is 350° C. or higher and the tempering parameter H is 8000 to 18000. By performing the tempering process so as to be within the range of, the joint strength is remarkably improved. Each step will be specifically described below. In addition, the composition of the steel plate to be used will be described later.

[First energization step]
First, as a first energizing step, a sheet assembly in which two or more steel sheets including at least one steel sheet having a C content in mass% of 0.280% or more and 0.700% or less are superimposed, The plate is sandwiched between a pair of electrodes and applied with a current value I ₁ (kA) while applying pressure.

In the first energization step, it is preferable to set the current value I ₁ (kA) and the energization time t ₁ (ms) so that a nugget is formed by spot welding to join all the steel plates forming the set. FIG. 8 schematically shows an example of a nugget formed when the first energization step is performed on a plate set in which two steel plates are stacked. As shown in FIG. 8, electricity is applied between the

electrodes

2A and 2B while the

electrodes

2A and 2B are pressed so as to sandwich the plate set in which the

steel plates

1A and 1B are superimposed in the plate thickness direction. As a result, a nugget 13 and a heat-affected zone (so-called HAZ) 14 are formed in the current-carrying portion between the

steel plates

1A and 1B, and the steel plates are spot-welded.
The current value I ₁ in the first energization step uses a current value that can obtain the desired nugget diameter, and when half the thickness of the total plate thickness is t (mm), the energization time t ₁ is from 10 t-5 to 10 t + 5 cycles (50 Hz ) and so on. Aiming for a nugget diameter of 4√t or more is good from the viewpoint of joint strength and avoidance of expulsion. More desirably, it is 5√t or more. In order to form such a nugget diameter of 5√t or more without causing expulsion, it is desirable to set the up slope before the first energization step. Moreover, before the first energization step, pre-energization may be performed with a current value lower than that in the first energization step.
Further, the pressing force of the

electrodes

2A and 2B against the plate assembly is, for example, 2000 to 8000N so as to suppress the occurrence of expulsion and stably obtain nuggets. The applied pressure may be constant or may be changed in the middle. In addition, if there is a variation in the applied pressure before the two-stage energization, the grain growth is hindered and the effect of regulating the grain size can be reduced. It is preferable that the variation in the pressure exerted by both

electrodes

2A and 2B on the set is small. With respect to the pressure P in the first energization process, the pressure in the first non-energization process is preferably 0.8P to 1.2P, and the pressure in the second energization process is 0.8P to 1.2P. Preferably. In the first energizing step, the first non-energizing step, and the second energizing step, it is more preferable to keep the pressure applied to the plate assembly by both

electrodes

2A and 2B constant.

Preliminary energization may be performed before upslope or nugget formation. A down slope or the like may be included. Nugget formation may be performed by pulse energization.

[First non-energized step]
After the first energization step, no energization is performed for a time _tc1 of 20 ms or more and 200 ms or less.
If the no-energization time _tc1 is less than 20 ms, the nugget edge may not solidify before the second current-carrying step. On the other hand, if the non-energization time _tc1 exceeds 200 ms, the nugget edge may become too solid before the second current application step.
In order to avoid post-energization (second energization step) in a state where the nugget edge is insufficiently solidified or excessively solidified, and to properly solidify the nugget edge (before the nugget edge is solidified or solidified) In order to avoid performing the second energization after the first energization step), the non-energization time _tc1 after the first energization step is 20 ms or more and 200 ms or less, preferably 25 ms or more and 160 ms or less, and 30 ms or more and 150 ms or less. is more preferable.

[Second energization step]
The second energization step is an important step in which the inventors of the present invention have found that the CTS can be improved even if the C content of the steel sheet is 0.280% or more. It has the effect of regulating the crystal grains in the vicinity of the fusion boundary in the nugget and improving the toughness of the portion where the stress applied in the peeling direction is the highest in the CTS test.
After the first non-energization step, the current is energized at a current value I ₂ (kA) that satisfies the following formula (1) and a time t ₂ (ms) that satisfies the following formula (2).
0.60≦I ₂ /I ₁ ≦1.10 (1)
50≦t ₂ ≦1000 (2)
In the second energization step, in order to melt the center of the nugget without crossing the melting boundary formed in the first energization step and apply appropriate heat to the vicinity of the nugget end, the current value (I ₁ ) in the first energization step The energization is performed under the condition that the ratio (I ₂ /I ₁ ) and the energization time (t ₂ ) satisfy the above formulas (1) and (2), respectively.
_The _second energization step corresponds to the grain control heat treatment, and the nugget crystal is The grain changes and joint strength can be improved.
I ₂ /I ₁ is preferably 0.75-1.05, and t ₂ is preferably 200-600.

[Tempering process]
After the second energization step, tempering is performed at the energized position.

Time: t _c2
In the tempering step, in order to temper the nugget formed by the first energization and the second energization, after the second energization step and before tempering, the temperature of the entire welded portion (nugget) is Ms point or less. need to be Therefore, the required time changes depending on the composition of the steel. FIG. 9 shows the time required to cool to the calculated Ms. The time (cooling time) _tc2 required for the temperature of the entire weld zone to drop below the Ms point was calculated by obtaining the Ms point from the above formula (4), and it was found that the following formula (3) must be satisfied.
t _c2 >3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100 (3)
The Ms in the formula (3) means the Ms point calculated by substituting the mass % of each element contained in the steel sheets forming the set into the element symbol in the following formula (4).
Ms (°C) = 561-474 x C-33 x Mn-17 x Ni-17 x Cr-21 x Mo (4)
In addition, among the elements in the formula (4), zero is substituted for the corresponding element symbols for the elements that are not contained in the steel sheet. In addition, if all the steel plates that make up the set of plates do not have the same composition, the Ms point calculated for each steel plate by Equation (4) for all the steel plates that make up the set of plates is The weighted average Ms point of the value obtained by multiplying the thickness ratio of each steel plate to the total thickness (total thickness) is substituted into Equation (3).

For example, in the case of a plate set in which three steel plates α, β, and γ having different compositions are superimposed, the Ms points (° C.) calculated by the formula (4) from the compositions of each steel plate are Ms _α , Ms _β , Ms _γ , t _α , t _β , and t _γ are the plate thicknesses (mm) of each steel plate, and t is the total thickness of the plate assembly, then the weighted average Ms point ( Ms _ave ) is calculated as follows.
Ms _ave =Ms _α ×(t _α /t)+Ms _β ×(t _β /t)+Ms _γ ×(t _γ /t)

If the cooling time _tc2 is too long, the fatigue strength in cross tension may decrease even if tempering is performed. A possible reason for this is that even if tempering is performed after cooling, a strong residual stress remains in the weld zone. Therefore, the cooling time _tc2 is preferably 9000 ms or less.

Tempering parameter: H
At the position where spot welding was performed by the first energization and the second energization, the tempering temperature is 350 ° C. or more after the time t _c2 (ms) described above has elapsed since the second energization was completed, and the following formula Tempering is performed under the condition that the tempering parameter H calculated by (A) is 8000 or more and 18000 or less.

H = T x (log _HT + (17.7 - 5.8 x [C])) (A)
T in the formula (A) means the tempering temperature (K) in the vicinity of the edge of the nugget formed by energization, _tHT means the tempering time (s), and [C] means the C content (% by mass) of the steel sheet. ) respectively. In the case of combining steel sheets with different C contents, the C content (% by mass) in the steel sheet with the highest C content is used.

The tempering parameter H is set to 8000 or more, preferably 9000 or more, and more preferably 10000 or more in order to proceed with the tempering sufficiently. Also, if the tempering proceeds too much, the carbides become too large and the toughness decreases.

If the tempering temperature T is too high, austenite crystallizes and the steel is quenched again. Therefore, tempering is performed below the transformation point. For this reason, the tempering temperature is preferably A _c1 (° C.) or lower, more preferably (A _c1 −30)° C. or lower, calculated by the following formula (B).
A _c1 =750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B (B)
The content (% by mass) of each element contained in the steel sheet is substituted for the element symbol in the above formula, and zero is substituted for elements not contained in the steel sheet.
In addition, in the case of a plate set that is not a combination of the same steel type (steel type with the same steel composition), the weighted average A _c1 according to the plate thickness, that is, for all the steel plates that make up the plate set, was calculated by formula (B) for each steel plate The tempering temperature can be set based on the weighted average _Ac1 of the value obtained by multiplying _Ac1 by the thickness ratio of each steel plate to the total thickness of the set.

Further, while the tempering temperature T in the formula (A) for calculating the tempering parameter H is absolute temperature (K), _Ac1 calculated by the formula (B) is in degrees Celsius (°C). Therefore, for example, when setting the tempering temperature based on A _c1 (°C) calculated by the formula (B) in the tempering process, the tempering temperature T (K) in the formula (A) is converted into an absolute temperature. , the tempering time t _HT (s) can be set such that the tempering parameter H is within a predetermined range.

In the present disclosure, the tempering temperature T (K) is based on the temperature at a position 0.5 mm inside from the nugget end after the second energization step (which may be referred to as “nugget end vicinity” in the present disclosure). and Here, the “nugget edge” is a portion of the fusion boundary of the nugget that was the plate interface of the plate assembly. In the case of tempering by a spot welder, it is difficult to directly measure the temperature near the edge of the nugget. For example, QuickSpot (Computational Mechanics Research Center Co., Ltd.) can be used as software for performing simulation by heat conduction analysis. In the examples described later, in the case of tempering by a spot welder, the temperature near the edge of the nugget was calculated by simulation using the above software. When a furnace or other heat source is used, the temperature in the vicinity of the temperature measuring section may be substituted, or the furnace temperature may be used.
Note that the tempering temperature T may change depending on the tempering means. In the present disclosure, the tempering parameter H is calculated as follows.

(1) When the temperature is constant If the tempering temperature in the tempering process is constant, each parameter is substituted into the equation (A) to calculate the tempering parameter H.

(2) When the temperature changes step by step Time: temperature t _HT0 to t _HT1 : T ₁ [K]
t _HT1 to t _HT2 : T ₂ [K]
…
t _HTk−1 to t _HTk : T _k [K]
then the tempering parameter H ₁ between t _HT0 and t _HT1 is
H ₁ =T ₁ ×(log(t _HT1 −t _HT2 )+(17.7−5.8×[C]))
is calculated as
Let t _HT2 ' be the time when this H ₁ is obtained at temperature T ₂ in the next section,
H ₁ =T ₂ ×(log( _tHT2 ′)+(17.7−5.8×[C]))
Then, the tempering parameter H ₁₊₂ from t _HT0 to t _HT2 until the second interval is
H ₁₊₂ =T ₂ ×(log( _tHT2 − _tHT1 + _tHT2 ′)+(17.7−5.8×[C]))
becomes.
Repeating this, H in all intervals is
H= _Tk *(log( _tHTk _-tHTk-1 + _tHTk ')+(17.7-5.8*[C]))
becomes.
When the tempering temperature changes stepwise, the tempering parameter H in the tempering process is calculated in this way.
Also, the temperature at which H obtained on the assumption that the entire section is isothermal is the same is called the representative temperature.

(3) When the temperature changes continuously A section is set so that the temperature changes within 50°C, and the average temperature _during that section is taken as the representative temperature of that section. By this method, the interval t _a to t _b is divided, and H is calculated by applying the method of “(2) When the temperature changes stepwise”.

Ha=T _ave ×(log(t _b −t _a )+(17.7−5.8×[C]))
Then, _tc is obtained from the next interval _tb , and H is calculated for the entire interval by the same method as in (2). Also, as in (2), the temperature at which H obtained by assuming that all sections are isothermal is called the representative temperature.

The tempering method in the tempering process is not particularly limited as long as the tempering temperature is 350°C or higher and the tempering parameter H calculated by the formula (A) is within the range of 8000 to 18000. After the second energization step, there are a method of performing tempering by a spot welder as it is, and a method of performing tempering using a heat source other than the spot welder.

<Tempering with a spot welder>
After the second energization, the energization is stopped while the electrode is pressurized, and then energization is performed again to perform tempering.
That is, after the second energization step, after cooling for a time t _c2 (ms) that satisfies the above-described formula (3) without energization, in the third energization step, the current value I ₃ ( kA) and the time t ₃ (ms) that satisfies the following formula (6).
_0.4≤I3 / _I1≤1.0 (5)
450≦t ₃ (6)
The third energization step corresponds to temper heat treatment, and the current value I ₃ and energization time t ₃ reheat the nugget cooled to the Ms point or less so that the tempering parameter H is within the range of 8000 to 18000. . As a result of the experiment, in the current value (I ₃ ) in the third energization step, the ratio (I ₃ /I ₁ ) to the current value (I ₁ ) in the first energization step and the energization time (t ₃ ) were expressed by the formula (5) ) and formula (6), the toughness can be effectively improved.
In addition, if the energization time in the third energization step is too long, the productivity is lowered, so it is preferable to set the energization time to 5000 ms or less.
After the third energization step, it is preferable to provide a so-called holding time during which only pressurization is performed and energization is not performed.
In this way, if tempering is performed by performing non-energization and energization while the electrodes are pressed against the plate assembly subsequent to the second energization step, the steps from the first energization step to the tempering step can be performed continuously. , work efficiency and productivity can be improved.
After the second energization, the electrode is once separated from the plate assembly, and after the time _tc2 has elapsed, the spot welder is used again to energize under the same conditions as in the third energization step to perform tempering. good too.

<Tempering by a heat source other than a spot welder>
Tempering may be performed using a heat source other than a spot welder. That is, after the second energization, the electrode is separated from the plate assembly, and the nugget is heated using a heat source other than the spot welder after the time _tc2 that satisfies the equation (3) has passed. A heat source (heating means) other than the spot welder is not particularly limited, and includes a furnace, laser, hot iron, hot plate, high-frequency induction heating, and the like. Heating is carried out so that the tempering parameter H is within the range of 8000 to 18000 when any heating means is used.
If a heating means other than a spot welder is used as a heat source for tempering, the variation in tempering temperature is reduced as compared with tempering by energization using a spot welder. When a spot welder is used, there are heat flows to nearby steel materials and branch currents to other welding points, so it is necessary to set a current value that incorporates these factors. On the other hand, the above-described heating means has few influencing factors and can easily obtain the target temperature, so that it has the advantages of high robustness and less labor for obtaining high joint strength.

In the method of manufacturing a welded joint according to the present disclosure, at least one steel plate constituting the set of plates may have a C content of 0.280% or more and 0.700% or less in mass%. The number of steel plates constituting the set of plates is not particularly limited as long as it is two or more, and may be selected according to the application of the welded joint to be manufactured. Arbitrary elements other than C, the plate thickness, the total thickness t of the plate assembly, etc. are as described above with respect to the welded joint, and the explanation here is omitted.

A plate set in which two or more steel plates including at least one steel plate having a C content of 0.280% or more and 0.700% or less are superimposed is subjected to resistance spot welding and baking consisting of the steps described above. By performing the return, it is possible to greatly improve the CTS compared to the case where the resistance spot welding is performed with a single energization.
Although the field to which such a method for manufacturing a welded joint according to the present disclosure is applied is not particularly limited, it is considered to be particularly effective, for example, in processes such as assembly of a vehicle body and attachment of parts.

Then, through each of the above steps, the welded joint according to the present disclosure, that is, the short diameter of the prior austenite grains in the melt boundary region from the melt boundary of the nugget end to 1 mm inside, which corresponds to the plate interface of the nugget part. The number density of iron-based carbides having an average ratio of the major axis to the major axis (major axis/minor axis) in the range of 1.0 to 1.5 and having an equivalent circle diameter of 30 nm or more in the fusion boundary region is 3.0 per 1 mm ² More than x10 ⁶ xC spot welded joints can be produced.
In Patent Document 3, tempering is performed at a relatively low temperature (220° C. or less), and even if the number density of carbides of 5 nm or more is 2×10 ⁶ /mm ² or more, carbides of 30 nm or more It is considered that the number density of is not sufficient, and the fine precipitation of carbides makes the joint hard, making it difficult to increase the joint strength.

In addition, the following (A) to (D) were clarified as shown in Examples described later.
(A) CTS can be improved even in high-strength steel sheets with a tensile strength TS (MPa) relative to the carbon content of 1800 × [C] + 250 or more, and the tensile strength TS (MPa) relative to the carbon content MPa) is 1800×[C]+250 or more, the effect of improving toughness in addition to the effect of improving CTS can be expected.
(B) When the holding time _tc2 is 9000 msec or less, the residual stress becomes small and the CTS improvement rate becomes high.
(C) Variation in CTS is reduced when the carbide precipitate density within 500 μm from the nugget edge in the HAZ is 1.0×10 ⁶ ×C or more per 1 mm ² .
(D) When a steel sheet having a C content of more than 0.30% is used, the effect of improving CTS is enhanced.

Hereinafter, the welded joint and the method for manufacturing the welded joint according to the present disclosure will be described with reference to examples. It should be noted that the welded joint and the method of manufacturing the welded joint according to the present disclosure are not limited to these examples.

A steel plate having the composition shown in Table 1 was prepared and subjected to resistance spot welding under the conditions shown in Table 2 (plate assembly, applied pressure, energization conditions, etc.) and tempering under the conditions shown in Table 3.

In Table 1, even if P, S, and N were not intentionally added, component analysis was carried out, and P: less than 0.010%, S: 0.0100% or less, and N: 0.0100% or less. there were. The balance is Fe and impurities. In addition, in Table 1, the values where the C content of the steel sheet is less than 0.280% or more than 0.700% are underlined. Underlines in Table 2 mean that the requirements of this disclosure are not met. However, the steel plate k and the steel plate l have a C content of less than 0.280%, but are used for the plate combination of the invention example combined with a steel plate having a C content of 0.280% or more and 0.700% or less. The steel sheets k and l are not underlined in the "Steel plate combination" in Table 2.

Underlines in Table 3 mean that the requirements of this disclosure are not met. "Only the first energization step CTS" means the CTS when the sample is produced only with the _first energization (I1, t1) _among the energization conditions, and may be hereinafter referred to as "single energization CTS".

The calculation of the tempering parameter H when the tempering temperature changes over time will be described using No. 9 in Table 3, which was tempered by spot welding, as an example. When the temperature history in the vicinity of the edge of the nugget was estimated by heat conduction analysis in the tempering process by spot welding, the temperature history shown in FIG. 10 was obtained. The average temperature was calculated within a range in which the temperature change did not exceed 50°C. FIG. 11 shows the average temperature change when segmented into ranges not exceeding 50°C. In that interval, the tempering parameter H was obtained by the method described above in "(3) When the temperature changes continuously".

The rate of increase calculated by the following formula was obtained in comparison with the single current CTS, and it was judged that those exceeding 15% had the effect of improving the joint strength.
Rate of increase [%] = [(CTS under the energization conditions of the present disclosure - CTS of single energization) / CTS of single energization] x 100

In the invention examples, resistance spot welding that satisfies the conditions of the present disclosure is performed on a plate assembly in which at least one steel plate has a C content of 0.280% or more and 0.700% or less by mass%, and both The rate of increase in CTS exceeded 15% compared to the case where resistance spot welding was performed by single current flow. As shown in Table 2, for example, numbers 21 to 30 use a plate set in which two steel plates a are superimposed, and the pressure, current, and time in the first energization step are all the same, but Table 3 3, there is some variation in "CTS only in the first energizing step". This is affected by the difference in electrode retention time (hold time).
On the other hand, in the comparative example, since none of the steel sheets C content, spot welding, or tempering satisfies the conditions of the present disclosure, the rate of increase in CTS compared to the case of performing resistance spot welding by single current was less than 15%, and in some cases the CTS was lowered.
In addition, in Nos. 6 and 16 (reference examples), the nugget edge vicinity temperature in the tempering process is less than 350 ° C., but the tempering parameter H is in the range of 8000 to 18000 by performing tempering for a relatively long time. A joint with a CTS increase rate exceeding 15% was obtained.

A steel plate having the composition shown in Table 4 was prepared, and resistance spot welding and tempering were performed under the conditions shown in Table 5 (plate assembly, applied pressure, energization conditions, etc.).

In Table 4, even if P, S, and N were not intentionally added, component analysis was carried out, and P: less than 0.010%, S: 0.0100% or less, and N: 0.0100% or less. there were. The balance is Fe and impurities. In addition, in Table 1, the values where the C content of the steel sheet is less than 0.280% or more than 0.700% are underlined. Underlines in Table 5 mean that the requirements of this disclosure are not met. However, steel plate H has a C content of less than 0.280%, but is prepared for use in a plate assembly of an invention example that is combined with a steel plate having a C content of 0.280% or more and 0.700% or less. Steel plate H is not underlined in Table 5, “Steel plate combination”.

Underlines in Table 5 mean that the requirements of this disclosure are not met. "Only the first energization step CTS" means the CTS when the sample is produced only with the _first energization (I1, t1) _among the energization conditions, and may be hereinafter referred to as "single energization CTS".
CTS was measured according to JIS Z3137:1999.

The "average ratio of the long diameter to the short diameter of the prior austenite grains at the nugget edge" and "the number of iron-based carbides of 30 nm or more per 1 mm ² at the nugget edge" were measured by the methods described above.
"E+" in the "3.0×10 ⁶ ×C" column of Table 6 means the factorial of 10, for example, "8.4E+05" means "8.4×10 ⁵ ".

In the invention example, a plate set in which the C content of at least one steel plate is 0.280% or more and 0.700% or less by mass% is used, and the ratio of the major axis/minor axis of the prior austenite grains at the nugget end (aspect Resistance spot welding and tempering are performed under conditions where both the ratio) and the number density of iron-based carbides are within the range of the present disclosure. rate was over 15%.
On the other hand, in the comparative example, any one of the C content of the steel sheet, the ratio of the major axis/minor axis (aspect ratio) of the prior austenite grains at the nugget edge, and the number density of iron-based carbides is outside the scope of the present disclosure, In some cases, the rate of increase in CTS was less than 15%, and in some cases the CTS was even lower than in the case of single current resistance spot welding.

<Examples A1 and A2>
The plate thickness of the steel plate Q is 1.6 mm. A set of two sheets of steel sheets Q was spot-welded, and then steel sheets Q1 and Q2 having different tensile strengths (TS) were obtained by changing the annealing conditions.
The applied pressure in spot welding is constant at 400 kgf, the first energization process has a current value of 7.5 kA, the energization time is 360 ms, the first non-energization time is 80 ms, and the second energization process has a current value of 7.0 kA and energization. Temper energization was performed at a time of 500 ms and a current value of 4.3 kA with an energization time of 1500 ms and a current value of 4.3 kA while pressure was maintained after 600 ms of discontinuation of energization after the second energization.

The joints thus obtained were subjected to a CTS test and the fracture toughness value of the weld was measured. For measurements, ``Recent Issues in Automobile Body Joining Technology and Countermeasure Techniques - Part 1'', Shin Nippon Steel Technical Report No. 393 (2012) and ``Fracture Mechanics Consideration of Spot Welded Joint Cross Tension Test (2nd Report) Spot Welding The method described in "Development of Evaluation Method for Fracture Toughness of Parts" (Shin Nippon Steel, Fuminori Watanabe et al.) was used. Specifically, a miniature CTS specimen (W=2 mm, B=1 mm) was cut from the nugget edge, a crack was pre-introduced, and a crack opening load was applied by using a wire. Then, the toughness value was estimated based on the load at the time of fracture. Table 7 shows the results. Note that the reference TS is a value calculated by 1800×[C]+250.

A2 (comparative example) has a low TS with respect to the carbon content. It is considered that this is because coarse carbides are formed. It is thought that the toughness of the welded joint was lowered due to the formation of coarse carbides, and the CTS of only the first energization is lower than that of A1 (Invention Example).
Furthermore, although there is an effect of energization in the second and third stages provided to improve CTS, the width is narrower than that of A1. Coarse carbide remained even after the second energization and became even coarser due to subsequent tempering.
As described above, it can be seen that a steel sheet having a TS that is not suitable for the amount of carbon cannot sufficiently improve the CTS.

<Examples B1 and B2>
A plate assembly in which two steel plates C in Table 4 were stacked was spot-welded. The applied pressure is constant at 3000 N, the first energizing step has a current value of 7.0 kA, the energizing time is 300 ms, the first non-energizing time is 40 ms, the second energizing step has a current value of 6.20 kA, the energizing time is 100 ms, and the second After 600 ms (B1) or 9500 ms (B2) of discontinuation of energization after the second energization, tempering energization was performed with an energization time of 1000 ms and a current value of 4.0 kA while pressure was maintained.

The joints B1 and B2 thus obtained were subjected to a CTS test. Furthermore, the residual stress was measured. The measurement method was described in "Simulation of welding residual stresses in resistance spot welding, FE modeling and X-ray verification" JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 205 (2008) 60-69. Specifically, for a diameter of 2 mm (the center of the nugget diameter), the X-ray diffraction angle 2θ is calculated using a value between 95 degrees and 105 degrees, the Young's modulus is 200 GPa, and the Poisson's ratio is 0.3. did. Table 8 shows the results. Threshold of residual stress: It can be judged that less than 90 MPa is preferable.
Comparing B1 and B2, the residual stress was smaller in B1, and the CTS improvement margin (improvement rate) was larger.

<Examples C1 to C7>
Variation in CTS is reduced when the carbide precipitate density within 500 μm from the nugget edge in the HAZ is 1.0×10 ⁶ ×C or more per 1 mm ² . Specifically, the nugget was cut in the plate thickness direction so as to pass through the center part, and an observation area of 0.25 mm ² was observed for the HAZ part within 500 μm from the nugget end in the cross section. The method for measuring the number density of coarse iron-based carbides in the HAZ is the same as the method for measuring the number density of coarse iron-based carbides in the nugget edge.
The evaluation was made by standard deviation when the CTS values of 30 specimens were assumed to be normal distribution. It was judged that the variation was small when the value was 0.20 kN or less. Tables 9 and 10 show the results.

<Examples D1 to D6>
A plate set in which two steel plates of the same number shown in Table 11 were stacked was spot-welded. The carbon content of each steel sheet is as shown in Table 11, and the other additive elements are Si: 0.3% and Mn: 0.9%. The plate thickness is 1.6 mm.
The pressure is constant at 400 kgf, the first energization step has a current value of 7.5 kA, the energization time is 400 ms, the first non-energization time is 100 ms, the second energization step has a current value of 7.0 kA, the energization time is 400 ms, After 1000 ms of discontinuation of energization after the second energization, tempering energization was performed with an energization time of 2000 ms and a current value of 4.0 kA while pressure was maintained. The joint thus obtained was subjected to a CTS test. Table 11 shows the results.

From the above results, when the C content exceeds 0.30%, the CTS can be further improved.

The disclosures of Japanese Patent Applications 2021-058351 and 2021-058352 filed on March 30, 2021 are incorporated herein by reference in their entirety. All publications, patent applications and technical standards mentioned in this specification are herein referenced to the same extent as if each individual publication, patent application and technical standard were specifically and individually indicated. captured by

1A,

1B Steel plate

2A, 2B Electrode 13 Nugget 14 Heat affected zone (HAZ)

Claims

A spot-welded joint of a plate set in which two or more steel plates including at least one steel plate having a C content of 0.280% by mass or more and 0.700% by mass or less are superimposed,
In the cross section in the plate thickness direction of the plate assembly passing through the center of the nugget, the length of the major diameter to the minor diameter of the prior austenite grains in the molten boundary region up to 1 mm inside from the molten boundary of the nugget edge corresponding to the plate interface. When the average ratio (major axis/minor axis) is in the range of 1.0 to 1.5, and the C content (% by mass) of the steel sheets constituting the plate set is C, the circle in the fusion boundary region A spot-welded joint in which the number density of iron-based carbides having an equivalent diameter of 30 nm or more is 3.0×10 6 ×C or more per 1 mm 2 .
However, if the C content for calculating the lower limit of the number density of the iron-based carbides is not the same for all the steel plates that make up the set, the C of each steel plate that makes up the set A weighted average of values obtained by multiplying the content by the plate thickness ratio of each steel plate to the total thickness of the plate assembly.
The spot-welded joint according to claim 1, wherein the C content of all the steel plates that make up the set of plates is more than 0.300% by mass.
When the C content (mass%) in the steel plate with the highest C content in the plate set is [C], the tensile strength (MPa) of the steel plate with the highest C content is 1800 × [C] 3. A spot welded joint according to claim 1 or claim 2, which is +250 or greater.
The number density of iron-based carbides having an equivalent circle diameter of 30 nm or more in a region within 500 μm from the nugget end of the heat-affected zone existing around the nugget end is 1.0×10 6 × per 1 mm 2 The spot welded joint according to any one of claims 1 to 3, wherein C or more.
The spot welded joint according to any one of claims 1 to 4, wherein the residual stress at the center of the nugget is less than 90 MPa.
A plate set in which two or more steel plates including at least one steel plate having a C content of 0.280% or more and 0.700% by mass or less are superimposed is sandwiched between a pair of electrodes in the plate thickness direction and heated. A first energizing step of energizing at a current value I 1 (kA) while applying pressure;
After the first energization step, a first non-energization step in which the time tc1 of 20 ms or more and 200 ms or less is not energized;
After the first non-energization step, a second energization step of energizing at a current value I 2 (kA) that satisfies the following formula (1) and a time t 2 (ms) that satisfies the following formula (2);
0.60≦I 2 /I 1 ≦1.10 (1)
50≦t 2 ≦1000 (2)
After the second energization step, after the time t c2 (ms) that satisfies the following formula (3) has passed, the tempering temperature is 350 ° C. or higher at the energized position, and the tempering temperature is calculated by the following formula (A). A tempering step of performing tempering under conditions where the tempering parameter H is 8000 or more and 18000 or less;
t c2 >3.5×10 −3 ×Ms 2 −3.3×Ms+1100 (3)
H = T x (log HT + (17.7 - 5.8 x [C])) (A)
A method of manufacturing a spot welded joint, comprising:
The Ms in the formula (3) means the Ms point calculated by substituting the mass % of each element contained in the steel sheets forming the set of plates into the element symbol in the following formula (4). However, if all the steel plates that make up the set of plates do not have the same composition, the total thickness of the set of plates is added to the Ms point calculated for each steel plate by the above formula (4) for all the steel plates that make up the set of plates. Substitute the weighted average Ms point of the value obtained by multiplying the plate thickness ratio of each steel plate to the formula (3).
Ms (°C) = 561-474 x C-33 x Mn-17 x Ni-17 x Cr-21 x Mo (4)
T in the formula (A) means the tempering temperature (K) in the vicinity of the edge of the nugget formed by the energization, tHT means the tempering time (s), and [C] means C content in the plate set. It means the C content (mass%) in the steel sheet with the highest amount.
The method for manufacturing a spot welded joint according to claim 6, wherein in the tempering step, the tempering is performed using a heating means selected from the group consisting of a furnace, a laser, a tempering iron, a hot plate, and high-frequency induction heating.
When the value calculated by the following formula (B) is A c1 (° C.), in the tempering step, the tempering is performed so that the tempering temperature T is (A c1 −30)° C. or less. A method for manufacturing a spot-welded joint according to claim 6 or 7.
A c1 (° C.)=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B (B)
The mass % of each element contained in the steel sheets forming the set is substituted for the symbol of the element in the formula (B). However, if all the steel plates that make up the set of plates do not have the same composition, A c1 calculated by the above formula (B) for each steel plate for all the steel plates that make up the set of plates is added to the total thickness of the set of plates. The above (A c1 -30) is determined based on the weighted average A c1 of the values obtained by multiplying the thickness ratio of each steel plate to .
The method for manufacturing a spot-welded joint according to any one of claims 6 to 8, wherein the C content of all the steel plates that make up the set of plates is more than 0.300% by mass.
When the C content (mass%) in the steel plate with the highest C content in the plate set is [C], the tensile strength (MPa) of the steel plate with the highest C content is 1800 × [C] The method for manufacturing a spot-welded joint according to any one of claims 6 to 9, wherein a spot-welded joint of +250 or more is manufactured.
The method for manufacturing a spot-welded joint according to any one of claims 6 to 10, wherein said t c2 is 9000 msec or less.
12. The method according to any one of claims 6 to 11, wherein in the first energizing step, the first non-energizing step, and the second energizing step, the pressure applied by the pair of electrodes to the plate assembly is constant. A method for manufacturing a spot-welded joint.